Time-Interleaved Digitizer Calibration Using Harmonic Mixing
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Solution Overview
Problem
Test and measurement instruments with built-in calibration oscillators that do not span the entire range of potential signal sources face limitations in calibrating systems, as they require access to a signal source that covers the full frequency range for accurate calibration, which is not always feasible, especially under varying environmental conditions.
Innovation Solution
The implementation of a compensation oscillator that is tunable over a wider frequency range than the internal ADC channel process uncertainty, allowing for real-time calibration and correction of phase and amplitude errors, and the use of a Look Up Table (LUT) to store filter coefficients for correcting hardware errors based on measured clock delay and skew values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a built-in calibration oscillator is used for calibration, then the calibration process can be performed with internal resources, but the oscillator cannot generate signals spanning the entire input bandwidth
Solution Approach 1:
The input bandwidth is divided into multiple frequency ranges, with each range calibrated using a specific harmonic of the compensation oscillator. The oscillator operates at a fundamental frequency and generates multiple harmonics (2f, 3f, 4f, etc.) that collectively span the entire input bandwidth, allowing segmentation of the calibration process across different harmonic frequencies.
Solution Approach 2:
The system changes the operating parameter of the compensation oscillator by utilizing different harmonic multiples (2x, 3x, 4x, etc.) of its fundamental frequency. By mixing the input signal with these harmonics and adjusting the mixing ratios, the system achieves calibration across a frequency range much wider than the oscillator's fundamental frequency, effectively changing the frequency parameter dynamically.
2Measurement precision
If hardware adjustments are made for mixing clock amplitude and phase, then interleaving errors can be reduced, but the calibration requires access to full frequency range signal sources
Solution Approach 1:
The system performs self-calibration using its own built-in compensation oscillator as the signal source. The compensation oscillator generates the calibration signals internally, eliminating the need for external full-frequency-range signal sources. The system uses its internal resources to characterize and correct its own hardware mismatches through the harmonic mixing process.
3Adaptability or versatility
If a tunable compensation oscillator is used to span the full bandwidth, then full frequency range calibration is enabled, but the oscillator complexity and cost increase
Solution Approach 1:
The system uses periodic sampling and time-interleaved ADC architecture where multiple ADCs operate in parallel with staggered timing. The compensation oscillator provides a fundamental frequency that, when harmonically mixed, enables periodic calibration across the full bandwidth. The harmonic mixing process naturally exploits the periodic nature of the oscillator signal to achieve wideband calibration without requiring the oscillator itself to be tunable across the full range.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables effective calibration and correction of hardware errors across the full input bandwidth, even when the internal compensation oscillator cannot generate signals spanning the entire bandwidth, ensuring accurate signal reconstruction and minimizing the impact of environmental changes on instrument performance.
Implementation Method 1
harmonic mixing for reducing noise
Data Source
AI summary
A test and measurement instrument includes a coefficient storage facility coupled to a programmable filter. The coefficient storage facility is configured to store at least two pre-determined filter coefficient sets, and configured to pass a selected one of the at least two pre-determined filter coefficient sets to the filter based on a measurement derived using a compensation oscillator. The measurement may include clock delay and clock skew. In some examples the test and measurement instrument may additionally adjust clock delay and/or clock skew in addition to selecting appropriate filter coefficients.